1. Field of the Invention
The present invention relates to a steering system for a multiple-axle vehicle having at least one front, one middle and one rear axle arrangement which are steerable via a steering rod/cylinder arrangement mutually and/or separately. Each axle arrangement may be formed by at lest one axle having two wheels, two axles having four wheels, and/or three axles having six wheels.
2. Description of the Prior Art
A demand exists for multiple-axle vehicles, for example trucks or crane vehicles, wherein the wheels, when cornering, generate the least possible friction on the road surface. The intention is to adjust the steered deflection (i.e., steering angle) of each wheel so that, where possible, the wheel rolls with hardly any movement lateral to its rolling direction. In the ideal situation, all wheels of the vehicle move in a circular path about a steering pole, which optimally is the sole point of intersection of the axes of rotation of all the wheels.
For trucks and similar vehicles designed for relatively fast and relatively light on road travel, but not for mobile crane vehicles (e.g., crane vehicles having more than four axles), steering systems have been developed which, with the aid of electronic controls, provide such an optimum steered deflection of all the wheels. Known in this case is an independent steering system in which the steering angles of the first and last axle are sensed by potentiometers. Via computer programs, the necessary steering deflections are established for the wheels of the remaining axles, and then the wheels are steered electro-hydraulically. No mechanical connections exists among the axles, i.e., the axles or wheels of an axle line are steered independently of each other.
To maneuver such vehicles in tight situations on the construction site and on the road, it is necessary to equip them with a separate steering device. In such separate steering arrangements, for example, the wheels of a front axle system are deflected more or less than those of the rear axle system; wherein the steering arrangements of the axle systems should permit independent adjustment.
From German patent application DE 41 19 641 A1, a device for selecting the steering of a mobile crane is known. This steering system includes a selector unit permitting decoupling of the steering of the front and rear axle arrangements, resulting in improved steerability for on road and/or off road operation.
German patent application DE 42 21 973 A1 discloses a steering system for a mobile crane intended to permit, on the one hand, a stable driving response when on road and, on the other, adequate wheel deflection in tight cornering. For this purpose, a gear segment lever is integrated in the steering rod arrangement, the toothing of which is designed so that in straight-ahead travel the steering effect is less than in tight cornering.
The disadvantage of separate steering devices known hitherto is that when, in tight cornering, one of the axle or wheel arrangements (e.g., the front or rear wheel arrangement) is deflected more than the other, the steering system goes "out of tune."
On a vehicle having a front, a middle and a rear axle arrangement the steering pole as defined above is displaced when the wheels of one of the axle arrangements are more strongly deflected than the wheels of the other axle arrangements. If, for instance, the rear wheels are deflected more than the front wheels then the axes of rotation of the front and rear wheels intersect at a point which is shifted toward the front of the vehicle. The axes of rotation of the wheels of the middle axle arrangement fail to continue to pass through this point, however, causing the wheels of the middle axles to scrub laterally on the surface of the ground resulting in friction.
One special movement often required of crane vehicles off-road, e.g., on site, is what is called "dog trotting" or "crab steering." In dog trotting, the vehicle moves sideways and to the front or rear without changing the direction of its longitudinal axis. The intention here is that all wheels having ground contact receive the same steered deflection. On conventional steering systems, however, this is not possible with all wheels since the steering kinematics also need to be designed for stable on road travel. Conventionally at least the middle axles need to be designed as rigid axles or as axles having only a slight deflection. Consequently, in "dog trotting," the edge deflection angles of the first and last axles greatly differ from those of the middle axles.
This is why the wheels of the middle axles are normally lifted from ground contact during dog trotting. Unfortunately, this results in higher axle loading of the first and last axles, and necessitates higher steering forces.